1. Field of the Invention
The present invention relates to a Cr-Mo steel for use as very thick plates of 75 mm or more for oil refinery, coal liquefaction and coal gasification equipment, and more particularly to a Cr-Mo steel of a novel composition which is free from lowering of its strength and toughness when formed into plates of extremely large thicknesses and hence is suitable for use as plates not less than 75 mm in thickness.
2. Description of the Prior Art
Heretofore, Cr-Mo steels of the type presented by ASTM (American Society for Testing Materials) A387-22 (21/4Cr-1Mo steel) and ASTM A387-21 (3Cr-1Mo steel) have widely been employed for medium- and high-temperature pressure vessels in chemical industry, including reaction vessels for oil refining such as desulfurization units and the like. In near future extremely thick Cr-Mo steel plates will probably be employed for gigantic equipment for coal liquefaction and gasification.
This type of Cr-Mo steel is usually given normalizing and tempering and its strength and toughness are maintained by a fine tempered bainite structure. But when the rate of cooling the plate from the normalizing temperature is slow due to its increased thickness, proeutectoid ferrite is formed to decrease its strength and toughness. In order to suppress the formation of the ferrite in the manufacture of Cr-Mo steel plates of extremely large thicknesses, it is general practice in the prior art to accelerate the cooling from the normalizing temperature by what is called quenching, whereby to retain the strength and toughness of the plates.
In the case of steel materials which are subjected to ordinary normalizing and tempering to give strength and toughness, there is no problem with workability, strain and so forth in the heat treatment which follows hot forming of the steel materials into pressure vessels and the like, such as tempering and normalizing-tempering. In contrast thereto, in the case of extremely thick steel plates which are subjected to quenching to give strength and toughness, however, re-quenching is required after hot forming and this introduces difficulties in respect of workability, strain and so forth. Accordingly, for the fabrication of pressure vessels from extremely thick steel plates, there is eager demand for the development of steel materials which are assured of strength and toughness by ordinary normalizing and tempering.
It is presumed that the Cr-Mo steel will be applied to the fabrication of coal liquefaction equipment from the viewpoints of strength, especially high-temperature strength and hydrogen attack resistance, and that the coal liquefaction equipment will be operated at higher temperatures and higher pressures so as to improve the efficiency of coal liquefaction. This will necessitate an increase in the thickness and strength of the steel plate for pressure vessels of the coal liquefaction equipment and hence will inevitably call for the manufacture of 300 to 400 mm thick steel plates of the bainite structure.
Such extremely thick Cr-Mo steel plates cannot be obtained by merely quenching Cr-Mo steels of conventional composition. With such a large thickness, the steel of conventional composition, even if quenched, is subject to precipitation of the proeutectoid ferrite and hence is not immune from lowered strength and toughness. Accordingly, in the forming of an extremely thick steel plate for the pressure vessel, the formed plate must be quenched overcoming the problems of workability, strain and so forth; hence, it is highly desired to develop proper steel materials which meet such requirements. Since the pressure vessels which are made from this kind of steel materials are mostly designed in accordance with ASME (American Society of Mechanical Engineers) standards of design or standards based thereon, however, the steel materials used are also limited specifically to those of ASTM specifications or similar ones and it is not permissible to make a substantial change in the composition of the steel materials or to add special alloying elements such as Ni, V, Nb and so forth to the steel materials to such an extent as to contribute to increasing their strength. Accordingly, it has been regarded as very difficult to develop steel materials of such a composition that satisfies the requirements for forming them into plates of extremely large thicknesses.